MadSci Network: Genetics

Re: Eye color and race? And genetic expression?

Date: Mon Mar 12 12:31:31 2001
Posted By: Yvonne A. Simpson, Grad student, Pathology, Edinburgh University
Area of science: Genetics
ID: 983583218.Ge

Dear Mark, 
This is quite a difficult one to answer.  Mendel's principles (Mendel of 
the sweet pea fame) were concerned the inheritance of genes but there is 
more to 
the story than that.  I think what you need to do is have a look at the 
subjects of Penetrance and expressivity.  Penetrance is the percentage of 
individuals with a particular genotype who have the phenotype associated 
with that genotype. If individuals have a particular gene they may still 
not express it if they have supressors, modifiers or epistatic genes in the 
rest of the genome.  Penetrance can be used to measure such effects when 
the cause of the modification are unidentified. 

Expressivity measures the extent to which a given genotype is expressed in 
the individual at the phenotypic level.  Factors which effect this may be 
due to the environemt or because of differences in the allelic contitution 
of the rest of the genome.  

These two things can often make it impossible to make predictions in human 
pedigree analysis.  You've really got to consider Mendel's inheritance and 
patterns of expression.  

You can have 
Complete dominance (Mendel's situation)Dominant homozygous and heterozygous 
look the same.  This is one extreme of a spectrum.  The other end of this 
spectrum is co-dominance .  You must remember that this is not evidence of 
blending though.  Incomplete dominance is where you get intermediate 
phenotype heterozygotes.  Some traits depend on multiple alleles e.g. 

Now, the term 'race' doesn't really mean much in genetics.  This is because 
in our ancestry it is impossible to know when peoples met etc.  It is 
better just to deal with individuals and their genes.  I tried to look this 
up for you but I coldn't find much about eye colour.  As far as I know 
brown eye alleles are dominant but I suspect there is more to this since 
that is never used as an illustration of Mendelian inheritance in the books 
I read.  Skin colour is polygenic so it would be difficult to predict from 
skin colour what colour a person's eyes would be - you're dealing with 
difference alleles which are independent.  Think how skin colour can chance 
in the sun and you'll see some of the complications.  I hope I've given you 
a starting point for further research.  This story simply isn't a simple 

Co- dominance

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